Currently I'm trying to extend my editor with a diagram page. My model is a graph consisting of nodes and edges. Both are managed in a ListProperty. I implemented everything like the architecture example, besides the root entity. The root entity manage both nodes and edges.

You have a list of edges defined twice. Once at the root of the model and again inside the node element. Those are two styles of edge representation. You typically want one or the other. If you look at the map sample, you will see how to implement either option... IMap.Routes (top-level) and IDestination.RailwayRoutes (part of node).